Riding the Muni in San Francisco costs $1.50 . Exact change. No bills.
They have change machines that take a 10 or a 20, and give you some $5 bills. They also have change machines that take a $1 bill and give you quarters.
They have no machines at all that take a $5 bill. They do have an attendant in the booth, whose job it is to tell a constant stream of tourists that there's no solution to their problem. (She suggests buying something for $1 from a nearby stand, receiving $4 in change. A 20% commission, in short, for opening a cash drawer.)
What's wrong with this picture?